Introduction

Regarding TKA, patient specific cutting guides (PSCG), which have the same fitting surface with patient's bones or cartilages and uniquely specify the resection plane by fitting guides with bones, have been developed to assist easy, low cost and accurate surgery. They have already been used clinically in Europe and the USA. However little has been reported on clinical positioning accuracy of PSCG. Generally, the methods of making PSCG can be divided into 3 methods; construct 3D bone models with Magnetic Resonance (MR) images, construct 3D bone models with Computed Tomography (CT) images, and the last is to construct 3D bone models with both MR and CT images. In the present study, PSCG were made based on 3D bone models with CT images, examined the positioning accuracy with fresh-frozen cadavers.

Background

Mobile-bearing (MB) total knee prostheses have been developed to achieve lower contact stress and higher conformity compared to fixed-bearing total knee prostheses. However, little is known about the in vivo kinematics of MB prostheses especially the motion of the polyethylene insert (PE) during various daily performances. And the in vivo motion of the PE during stairs up and down has not been clarified. The objective of this study is to clarify the in vivo motion of MB total knee arthroplasty including the PE during stairs up and down.

Background

Mobile-bearing (MB) total knee prostheses have been developed to achieve lower contact stress and higher conformity compared to fixed-bearing total knee prostheses. However, little is known about the in vivo kinematics of MB prostheses especially about the kinematics of polyethylene insert (PE). In vivo motion of PE during squatting still remains unclear. The objective of this study is to investigate the in vivo motion of MB total knee arthroplasty including PE during squatting.

Recently mobile-bearing total knee arthroplasty (TKA) has become more popular. However, the advantages of mobile bearing (MB) PS TKA still remain unclear especially from a kinematic point of view. The objective of this study was to investigate the difference and advantage in kinematics of mobile baring PS TKA compared with fixed bearing (FB) PS TKA.

Femorotibial nearest positions for 19 subjects (20 knees), 10 knees implanted with NexGen Legacy flex (Zimmer, Warsaw, IN)with mobile bearing PS TKA, and 10 knees implanted with NexGen Legacy flex (Zimmer, Warsaw, IN)with fixed bearing PS TKA were analyzed using the sagittal plane fluoroscopic images. All the knees were implanted by a single surgeon. All the subjects performed weight bearing deep knee bending motion. We evaluated range of motion, axial rotation of the femoral component, AP translation of medial and lateral sides.

The average range of motion between femoral component and tibial component was 119°±18° in MB and 122°±10 ° in FB. The axial rotation of the femoral component was 11.8°±6.2° in MB and 11.8°±4.9° in FB. There was no significant difference both in range of motion and axial rotation between MB and FB. The AP translation of MB and FB showed same patterns. They were rollback in early flexion, the lateral pivot pattern (the medial condyle moved forward significantly compared with the lesser amount of AP translation for the lateral condyle) at mid flexion, and bicondylar rollback at deep flexion. The rollback in early flexion was 3.4mm in MB and 1.8mm in FB at medial side, 4.2mm in MB and 4.8mm in FB at lateral side. There was no significant difference. The lateral pivot pattern, which moved anteriorly, was 7.8mm in MB and 7.0mm in FB at medial side, 3.0mm in MB and 2.4mm in FB at lateral side. There was no significant difference. The bicondylar rollback at deep flexion was 6.4mm in MB and 7.7mm in FB at medial side, 6.9mm in MB and 4.8mm in FB at lateral side. In four subjects, more than 12°axial rotation was observed in knees implanted with FB TKA which allows only 12°axial rotation.

The results in this study demonstrate that there was no significant difference in kinematics of weight bearing deep knee bending motion between MB and FB. The advantage of MB is allowance of axial rotation which restricted until 12° in FB NexGen Legacy flex PS TKA.

Introduction: Recently several retrieval reports of PS TKA have demonstrated that the wear and deformation can occur on the anterior tibial post and the hyperextension of femorotibial components was a pivotal factor in the mechanism of anterior tibial post impingement. The objective of this study was to investigate the in vivo mechanism of anterior tibial post impingement during gait in PS TKAs.

Methods: Twenty knees with PS TKAs implanted by single surgeon were assessed in this study. The Review Board Committee of the author’s institution approved this study and informed consent was obtained from all patients. In this study ten knees implanted with Scorpio NRG PS (Stryker Orthopedics, Mahwah, NJ) and ten knees implanted with NexGen Legacy-flex fixed (Zimmer, Warsaw, IN) were examined. Each patient was asked to perform treadmill gait under fluoroscopic surveillance in the sagittal plane. Treadmill gait speed was 0.5–1.0 m/s at Patients selected comfortable speed. Patients were explicitly encouraged to fully extend their knees at heel-strike and to avoid a shuffling gait pattern. Patients used light-touch hand-support to maintain the foot position on the treadmill. In vivo 3D poses of the knee prostheses were computed using a two- to three-dimensional (2D/3D) registration technique, which uses CAD models to reproduce spatial postures of the femoral and tibial components from calibrated single-view fluoroscopic images. We evaluated range of motion and the anterior-posterior (AP) translations between femoral and tibial inserts. The anterior tibial post impingement was determined when the proximity between tibial post and femoral cam is within the 0.5mm threshold.

Results: The maximum flexion during gait was 41.9° (25°–56°) in NRG and 42.3° (23°–59°) in Legacy-flex. The minimum flexion during gait was 1.5° (−9.2° − 8.4°) in NRG and 1.8° (−13.0° − 17.0°) in Legacy-flex. The AP translations of the medial and lateral contact points in stance phase were significantly larger in Legacy-flex compared with NRG (medial ; p=0.02,lateral ; p=0.007, Mann-Whitney’s U test). Anterior tibial post impingement was recognized in four knees implanted with Legacy-flex, and in two of three knees, the knee was not hyper extended. On the other hand, no impingement was recognized in knee implanted with NRG.

Discussion: In this study, the anterior tibial post impingement occurred not only in hyper extended knee but also in slightly flexed knee in Legacy-flex. One of the reasons why the anterior tibial post impingement was recognized in Legacy-flex was the large amount of the AP translation in stance phase. Posterior translation in stance phase may be one pivotal factor in the mechanism of the anterior tibial post impingement.